William Einthoven in 1903. For this great achievement, he was awarded by a Nobel prize in 1924.
The electrocardiograph is an instrument which is used in the examination of people suffering from heart disease. It indicates how serious is their illness and maintains a check on recovery subsequent to a heart attack. In essence it detects the amount of electricity which passes through the heart muscle as it pumps blood through the body.
Einthoven’s successful invention stemmed from the discovery by two Germen scientists in the mid-19th century, that a frog’s heart creates an electric current.